Light sources or illumination devices consisting of light emitting diodes (LEDs) are increasingly used for replacing conventional light sources such as incandescent lamps and fluorescent light sources. LEDs offer many advantages compared to conventional light sources, especially when it comes to light conversion efficiency. However, one disadvantage is that LEDs generate light in a relatively narrow spectral band.
In many applications, such as retail applications and environments, the standard with respect to for example color rendition is set by light sources such as the Philips CDM-Elite. The CDM-Elite lamp has high quality of light and an excellent white rendition. The term “white rendition” refers to a desired improved white appearance of a white object being illuminated by a light source. When LED-based light sources are used to replace traditional lighting systems, they are especially required to generate light that is perceived as white.
Color reproduction is typically measured using the color rendering index (CRI), which is calculated in Ra. The CRI is sometimes also referred to as color rendition index. The CRI is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Daylight has a high CRI, where Ra is approximately 100. Incandescent bulbs are relatively close with an Ra greater than 95 and fluorescent lighting is less accurate with an Ra of typically 70-90.
Consequently, in order to achieve the desired “white” light in LED-based lighting applications, light sources with a high CRI are desirable. For LED lighting systems there are warm-white or neutral-white LED modules with a color rendering of about 80-90 readily available. Although the color rendition of these sources is good, white objects viewed under these light sources appear somewhat less white compared to the standard conventional alternatives. In some applications, this may be a disadvantage for these LED modules, especially for the mentioned retail applications where lamps having excellent white rendition, often referred to as “crisp white”, are preferred.
The term chromaticity is used to identify the color of a light source regardless of its brightness or luminance. In particular, the chromaticity of a light source can be represented by chromaticity coordinates, or color points in the 1931 CIE Chromaticity Diagram or the 1976 CIE Chromaticity Diagram (Commission International de l'Eclairage). The color temperature of a light source is defined in terms of an ideal, purely thermal light source also known as a blackbody radiator, whose light spectrum has the same chromaticity as that of the light source. The color temperature is measured in Kelvin (K). The so called black body locus (or line) is the path or line that the color of an incandescent black body would take in a particular chromaticity space as the blackbody temperature changes.
Many attempts have been made relating to improvement of the white light illumination of LED sources. US 2007/0284563 discloses a light emitting device including at least three different LEDs to emit light in the blue, green or red wavelength range, optionally a fourth LED having a peak emission wavelength of 410-460, and also including a wavelength conversion means to convert primary light into secondary light. Using LEDs having narrow emission ranges and a phosphor emitting yellow-green or orange light having relatively wide emission wavelength ranges high color rendering ability is achieved. According to US 2007/0284563, a high CRI (>90) can be achieved.
However, although the light is generally perceived as white and has good color rendering, the white light is still not the “crisp white” desired for many applications. Hence, there is still a need for an efficient solution for providing the desired white light having improved white rendition in LED applications.